Quantum Dots On Two Dimensional Graphene-Like Layered Materials

As one of the most promising candidates for quantum computer, quantum dots attract a variety of attentions of researchers. In this thesis, we performed low-temperature transport measurements to investigate the properties of quantum dots made by two-dimensional graphene-like layered materials. Compared with traditional semiconducting materials, two-dimensional graphene-like layered materials have the advantages of an atomically-thin geometry, inherent flexibility and dangling-bond-free interfaces, making them easy to integrate with various substrates. We’ve investigated quantum dots on two types of layered materials, graphene and transition metal dichalcogenides. The main content of the thesis includes:1. A brief introduction to quantum computation and its physical implementation. We introduced the constant interaction model, which is the basic theory for quantum dots. Using this simple model, we explained the basic experimental phenomena such as Coulomb blockade, Coulomb oscillations, Coulomb diamonds of single quantum dot, and honeycomb charge stability diagram of double quantum dots. Finally, the concepts of quantum point contact (QPC), photon assisted tunneling (PAT), relaxation time, and decoherence time have been introduced.2. An introduction to the equipment used in fabricating quantum dots on two-dimensional layered materials. Processing steps and some tips of fabrication have been introduced. Finally, we introduced the equipment for obtaining low temperature and electronic instruments for measurements.3. A brief introduction to basic properties of graphene and its identification on Si/SiO2 substrate. We demonstrated both single dot and double dots on graphene. The Coulomb diamonds and honeycomb diagram have been observed. In order to investigate the coherency of graphene quantum dots, we performed the noise measurements on graphene nano-device. By suspending the device over the substrate, we can track the origin of low frequency 1/f noise. We compared the noise level from the suspended graphene nano-device with that from unsuspended ones. Furthermore, by comparing our results with the noise level obtained from quantum dots on traditional semiconductor and graphene field effect transistors, we think the larger noise found in graphene nano-device is dominated by the edge states induced by etching steps. We presented a simple model to explain the experimental results.4. A brief introduction to basic properties of transition metal dichalcogenides and their identifications on Si/SiO2 substrate. We fabricated a field effect transistor on WS2, where characterized on-off curve was demonstrated both for back gate and top gate. A gate-defined quantum dot made on WSe2 has been fabricated, where Coulomb diamonds and Coulomb oscillations were found. The size of the quantum dot can be tuned by the voltages applied to top gates, which is consistent with the results simulated by COMSOL. We investigated Coulomb oscillations on WS2 quantum dot made in a similar way as well. The temperature dependence of Coulomb peaks is consistent with standard theory of quantum dot, no matter they are related to a quantum dot or an impurity trap on WS2. This result, which is different from graphene, shows the absence of a disordered confining potential, which relates to the larger noise level found in graphene nano-devices.The main innovations of this thesis are:1. Fabricating suspended graphene nanoribbon devices, where Coulomb blockade and Coulomb oscillations were found. For the first time, we measured the noise level of graphene nano-devices, both suspended and unsuspended.2. Comparing the noise level of suspended graphene nano-devices with that of unsuspended ones, we demonstrated for the first time that the noise is dominated by the edge states, rather than substrate.3. For the first time, we fabricated a gate-defined quantum dot made on WSe2. Coulomb diamonds have been found on the WSe2 quantum dot, which is isolated from edge states. The size of the quantum dot can be tuned by the voltages applied to top gates, which is consistent with the simulation.4. For the first time, we fabricated a quantum dot made on WS2. A detailed investigation of Coulomb oscillations on WS2 has been performed for the first time. Different from graphene, the disordered confining potential is absent in our designed WS2 quantum dot.